1. Field of the Invention
This invention relates to refrigeration systems, and more particularly to heat pump and air conditioning units that include an automatic defrost cycle.
2. Description of the Related Art
In FIG. 1 is an illustration depicting the operation of a heat pump unit operating in heating mode. Refrigerant cool vapor is transmitted through outdoor coils, also called an evaporator, and delivered to a reversing valve. The reversing valve is switched to the heating mode position so that the cool vapor is delivered to a compressor, which pressurizes the refrigerant and converts it into a hot vapor. The hot vapor refrigerant is then delivered to a set of indoor coils, also called a condenser, where it releases its latent heat to the room.
The warm liquid refrigerant leaves the condenser and then flows through a bypass valve and into a main liquid line. The main liquid line delivers the warm liquid refrigerant through a second expansion valve, where it expands and vaporizes and gains latent heat from the outside air. The cool refrigerant vapor from the outdoor coils then travels through the reversing valve and returns to the compressor where the cycle begins again.
It is well known that heat pumps can operate in both a cooling mode and a heating mode. For example, FIG. 2 is an illustration of a heat pump operating in a cooling mode in which the hot vapor refrigerant exits the compressor at a temperature in the range of 120–140° F., and is transferred to the outdoor coils, called a condenser. When operating in a cooling mode, the hot vapor refrigerant enters the condenser where it looses heat and condenses. The warm liquid refrigerant leaves the condenser, travels through a by-pass valve, and enters an expansion valve that regulates its flow so that it can be completely vaporized in the indoor coils, called an evaporator. The pressure drop through the second expansion valve vaporizes some of the warm liquid refrigerant and lowers its temperature to 40–50° F. As a result, it spontaneously gains more heat. The low-pressure refrigerant vapor leaves the evaporator, travels through a reversing valve, and returns to the compressor, where the cool vapor refrigerant in transformed into hot vapor refrigerant. The cycle then begins again. The rest of the continuous supply of warm liquid refrigerant is vaporized by picking up latent heat from the inside air as it passes through the evaporator's coils.
It is well known that during cold weather, ice and frost build up on the evaporator of a heat pump when operating in a heating mode. If the build up of ice and frost continues and is not removed from the evaporator, the efficiency of the heat pump is gradually reduced.
Heat pumps used in the prior art have a defrost cycle that removes ice and frost on the evaporator by reversing the direction of the hot vapor refrigerant through the coils similar to the flow of refrigerant shown in FIG. 2. These systems are known as ‘hot gas defrost systems’.
One important drawback with ‘hot gas defrost systems’ is that the unit's primary heating cycle must be reversed during the defrost cycle. When this occurs, not only is heat no longer added to the building, but heat from the warm air located inside the building is transmitted outside the building. In order to overcome the loss of heat from the building during the defrost cycle some buildings have secondary heating units. Unfortunately, these secondary heating units add to the overall cost of the heating and cooling systems.